Background & Aims: Multiple sclerosis (MS) is one of the chronic autoimmune diseases of the central nervous system with unknown etiology. The present study aimed to investigate the apoptosis and nitric oxide (NO) production of endothelial cells treated with serum of patients with MS and response to interferon beta (IFN- ) therapy. Methods: Human umbilical vein endothelial cells were treated with sera from patients with active MS (in relapse), MS in remission, or sera from healthy volunteers (each n = 10). Nitric oxide (NO) levels were determined in culture supernatants by Greiss method and endothelial cell apoptosis was assessed by annexin V-propidium iodide staining. Effects of IFN-beta-1b on endothelial cell apoptosis and NO production were tested at increasing doses (10, 100, and 1000 U/ml). Results: Compared with healthy people, only apoptosis of endothelial cells treated with serum of patients with relapsing phase increased, P<0.01; while there was no significant difference between apoptosis of endothelial cells treated with serum of patients in remission phase and healthy controls. Apoptosis of endothelial cells treated with sera of patients in relapse was decreased by IFN-beta-1b at 10 U/ml, P<0.05. The same dose also led to a significant increase in nitric oxide production. Conclusion: The results suggest that endothelial cells injury and apoptosis may play a role in MS etiology and represents a potential therapeutic mechanism of action for IFN-beta-1b in MS therapy.